The only performance guarantee that you have concerning the Internet is that performance will vary wildly from one time to the next. Performance-critical systems are not suited to becoming Web services.
Web services rely on HTTPthe same communication protocol upon which Web pages are requested and delivered. HTTP was designed to enable one server to handle hundreds of requests quickly by not maintaining a long-term stateful connection between the clients and the server. Instead, HTTP initiates a fresh connection with the server and maintains it only as long as it needs to transfer the data. Once complete, the connection is terminated and the server is then freed up to process a request from someone else. The server typically will not maintain any sort of concept of state to keep track of from one user's request to the next, although this can be achieved though the use of session tracking or cookies. This tends to make HTTP very transactional in nature.
Although the HTTP communication transaction enables the server side to handle many clients, it also means that a lot of time is wasted creating and terminating connections for clients that need to perform a large number of calls between the client and the server. Other technologiessuch as DCOM, CORBA, and RMIdon't have this problem because they maintain the connection throughout the application lifecycle.
See Hour 4, "Comparing Web services to Other Technologies," for more information on the differences between Web services and these other technologies.
The other performance consideration that must be taken into account with Web services is the conversion to and from XML during the communication process. All communication takes place as XML messages encoded in a special format (a SOAP envelope). This conversion takes time. Depending on how complex your data is and how much of it there is, this time could be a serious penalty. For instance, if you try to send a binary image as a method parameter, that data must be encoded into a format that can be represented in XML. This certainly is not as fast as transferring the image across the wire in its original format.
Although this sort of time penalty occurs with other architectures such as RMI and CORBA, it can be especially bad for Web servicesbased systems because data is transferred as XML textwith a large amount of extra information required in the SOAP envelope. This overhead isn't too bad when we consider a simple string or number to be encoded. Binary data, such as images, tend to be much larger though and take a lot longer to convert. XML tends to take more bytes to encode data than the equivalent binary representation.
Finally, when writing a Web service, you typically have a choice of many different languages and tools with which to build your solution. It is important that you pick ones that will scale to handle the expected loads adequately.